This invention relates to blow molding of oriented hollow articles of thermoplastic material and more specifically to an oven for heating the parisons to a predetermined target or orientation temperature.
In recent years, there has developed techniques whereby blow molded articles can be produced having biaxial orientation, which articles have exceptional strength and highly desirable physical properties such as clarity. This technique involves forming of parison, cooling it to well below its melting point, and thereafter heating it to orientation temperature. The orientation temperature is just below the crystalline melt point in the case of crystalline materials and 40.degree. to 225.degree. F. below the homogeneous melt point for amorphous materials. Such techniques are disclosed in U.S. Pat. Nos. 3,288,317 and 3,390,426.
Ovens for heating such parisons can be found in U.S. Pat. Nos. 3,752,641 and 3,740,868, both of which are assigned to the present assignee. In ovens of this type, parisons are disposed on pin structures carried by a movable chain through the oven. Parisons were carried along a plurality of generally parallel paths in an upright position and hot air was forced from the bottom of the oven and exhausted through the top. An improvement was made in this type of oven in U.S. Pat. No. 3,801,623 and assigned to the present assignee in which an oven of this same general construction was utilized whereby the heated air was blown across the oven from one side to another for more effective heating. Two specific problems occurred: (a) it took considerable time to heat certain materials, such as polypropylene, and (b) there was a tendency for the outer parison wall in an axial direction to be hotter than the corresponding inner parison wall in an axial direction resulting in the formation of a nonuniform container.
What is desired then is a parison oven which can reduce the time needed for heating parisons and thereby increase production thereof, and an oven for more uniformly heating the parisons so that the peripheral walls of the parisons have a temperature approximately equal to corresponding portions on the interior walls of the parisons.